Introduction

HEDMs are chemicals which have high enthalpies of formation. This means that their reactions generate large amounts of energy, as the thermodynamically unstable compound decomposes into more stable compounds. They are used for several purposes:
However, to be useful, an HEDM  must have more then a high enthalpy of formation. As well as being thermodynamically unstable, it must also be kinetically stable, as it must be transportable, and storable - an HEDM which decomposes five minutes after creation is no good to anyone! Also, it helps if it is highly dense, as the higher the density, the higher the energy density is, and  the easier it is to transport.

Explosives and propellants have additional requirements. They must react quickly under a fairly easily achieved condition, as their effectiveness depends on their speed of reaction. This requires their being dense, as the speed of the shock wave that causes detonation is proportional to the square of the density of the explosive. The products of their reactions must also have a much greater volume than the reactants - ideally, what is wanted is  a solid that decomposes to give multiple moles of gas. Because much gas needs to be generated, many explosives and propellants are nitrogen and/or oxygen rich. Some well-known HEDMs are shown below:

                                                                                                                     
                                            TNT                                                      Nitroglycerin                                                      Potassium  Chlorate (VII)
                         (1-methyl, 2, 4, 6-nitrobenzene)                           (1, 2, 3-trinitratopropane)


Cubane

Cubane is a hydrocarbon with the molecular formula C8H8. It, perhaps unsurprisingly, is formed with each carbon being at the vertex of a cube. It had long been  thought of as a potential HEDM before anyone had ever synthesised it - indeed, many thought it would be so strained that it would be impossible to make! With its very strained  bond angles, it can easily be seen why cubane's existence was thought impossible. However, cubane was synthesised, and proved to be very stable at room temperature, despite having an enthalpy of formation of +620 kJmol-11. One synthetic route2 for the synthesis of cubane is shown below:







Cubane is surprisingly stable; it has a thermal decomposition energy of 180kJmol-11 in the gas phase at 230-260oC. This  is because there are no kinetically viable paths along which cubane can decompose1. This can be seen quite easily by comparing cubane to a similar molecule - cyclobutane. Cubane consists of  6 cyclobutyl rings, so this is a fair comparison. The decomposition can start to occur one of two ways - with one bond breaking, or with two bonds breaking.


This decomposition route forms a diradical, an intermediate of high energy.  However, in cubane, this cyclobutyl ring will still be held in place by the others, i.e. a hugely reactive intermediate will be formed, and no strain will be released. Hence, this decomposition route cannot occur


This reaction is a 2+2 cycloelimination, and is therefore symmetry forbidden according to the Woodward-Hoffmann rules. This decomposition route cannot occur.

However, cubane, like other  hydrocarbons, does not react especially quickly - it's a fairly good fuel (if you ignore its exorbitant price), but it's not very useful as an explosive or a propellant. It also does not have as high an enthalpy of formation as other modern HEDMs. However, like other hydrocarbons, it can take other substituents, and these cubane derivatives can potentially be much higher in energy than their parent molecule.


Nitrocubanes
 Isocyanocubanes
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